Proof that the Oort Cloud Exists?

Question: I’m somewhat skeptical about the existence of the Oort Cloud. Some of the estimates I’ve heard indicate that it could stretch halfway to Proxima Centauri. Unless the interstellar medium is itself far less “empty” than predicted, it would seem unlikely that our star would have such a cosmic structure surrounding it that other stars lacked. Do we have any direct observational evidence that would confirm the existence of the Oort Cloud (evidence not explained by other theories)? Is it possible that the interstellar medium is simply less empty than predicted, and Oort-Cloud-like filler is common in interstellar space?  – Tim

Answer: There is what one could call “indirect observational evidence for the Oort Cloud.  It has been known since 1932 (first proposed by Ernst Opik, then updated by Jan Oort in 1950) that one needs a source for long-period comets that is beyond the orbit of Pluto.  This source of long-period comets, which are gravitationally-bound to our Sun, cannot be interstellar.  Also, we have seen Kuiper Belts (debris orbiting at distances from 30 to 50 AU) around other stars, and as the Oort Cloud is likely a continuation of the Kuiper Belt around our Sun, these properties appear to be common remnants of the star formation process.  Note that our abilities to detect small bodies (a few to 10s of kilometers in diameter) in the Kuiper Belt and Oort Cloud regions are improving, using techniques such as background star occultation, making the ultimate characterization of the Oort Cloud inhabitants in the next few decades a good possibility.

Jeff Mangum


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Advanced Rocket Propulsion Technologies for Long-Range Space Travel?

Question: Asking a theoretical question, in your own personal opinion what would be the most practical method of launching a probe to a nearby star (say, within 10 light years). Would our best bet to be launching a conventional chemical-rocket propelled probe as fast as we could manage on a gravity slingshot and waiting a century or so? Would some other option (which may or may not have been operationally built yet) within our current technological means seem like a better bet?

To be clear, asking for your opinion, not the current consensus (if one exists on the subject).

– Tim

Answer: At the present time, conventional chemical-rocket propelled space probes which use gravity-assist are the mode of choice for long-distance space travel.  I am not aware of any long-range propulsion technologies currently being considered or under development that could provide an improved efficiency for such long-range space travel.

Jeff Mangum

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Can a Black Hole Become Unstable and Produce a “Small Bang”?

Question: Can a black hole become unstable and cause a “small bang”? Creating parallel universes? – Raoule

Answer: I don’t believe that there are any reliable theoretical calculations that produce this series of events.  In fact, I don’t believe that “unstable” black holes lead to explosions of any kind.

Jeff Mangum

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A Question About a Career in Astronomy

Question: I am in 12th grade and am really very confused how to opt for astronomy and how?  [For example,] what should I do after 12th [grade]?  [For example]:
1.What to opt after 12th to enter in field of astronomy?
2.i want to be an astronomer? so which university should i apply for?
– Palavi

Answer: You should look through the posts on my Careers in Astronomy section of this blog for quite a few answers to questions very similar to yours (and more!).  I am sure that you will find answers to your questions there.  If not, please let me know.

Jeff Mangum

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What Equipment Does One Need to Send a Radio Signal to a Distant Star?

Question: Hi. I’m doing some research for a piece of fiction and was wondering: if you had a radio transmitter/receiver aimed at a far star 1) what kind of equipment would one need and 2) how would the equipment’s behaviour vary from a conventional satellite receiver? I know it’s not quite an astronomy question and more a question of radio astronomy technology, but I do hope you might be able to help. Any information would be very much appreciated.  – Steve

Answer: As a prelude to the answer to this question, let me dispel a myth about the detectability of Earth’s radio transmissions.  Even though it is true that many of our radio and television broadcasts are “leaking” into space at the speed of light, those signals are likely too weak to be detectable by any of the inhabitants of planets orbiting even the most nearby stars.  This is due to the fact that the strength of a light signal, such as a radio broadcast signal, decreases as the inverse-square of the distance that it travels from its source.

To illustrate this inverse-square law let us assume that we transmit a radio signal with a power of one megawatt (1 MW) into space.  For reference, the Arecibo Observatory has a radar system that it uses to map the surfaces of the Moon and nearby planets that transmits a signal power of about 1 MW at a frequency of about 2.4 GHz.  Let us further assume that the signal is transmitted at a frequency that can penetrate the Earth’s atmosphere and travels unimpeded through space.  This 1 MW signal would be just barely detectable at the nearest star, Proxima Centauri, which is at a distance of 4.243 light years.  Furthermore, keep in mind that this distant civilization would have to be staring at Earth with its receiving system tuned to exactly the same frequency that we are transmitting.  Therefore, it takes a rather power transmitter to send a detectable radio signal to the nearest star.  This sort of equipment is far more sophisticated than your average home satellite signal receiving system.

Furthermore, as you can probably imagine, this scenario has been investigated by quite a few researchers.  There are many factors that affect the detectability of radio signals emitted or transmitted from the Earth, complicating the rather simplistic scenario discussed above.  For a nice summary of these investigations see the io9 article on the pros and cons of sending beacon signals into space.

Jeff Mangum

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Inconsistency Between the Age and Diameter of the Universe?

Question: I have read that the diameter of the universe is 96 billion light years.  How can that be if the universe is a mere 14 billion years old?  Am I to conclude that the 96 billion figure is some extrapolation based on rapid inflation?  – Carlton

Answer: One of my physicist colleagues, Frank Heile from Stanford University, has provided an excellent answer to this question.  First of all, the 13.8 billion light years is derived from the radius of a sphere of the Cosmic Microwave Background (CMB) radiation that is being observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites.  These satellites have mapped the structures which are precursors to galaxies and clusters of galaxies.  Second, the 93 billion light year diameter estimate refers to the “observable” universe.

Now, if we waited another 46.5 – 13.8 = 32.7 billion years, we should actually be able to see the light emitted right now from those superclusters of galaxies which formed from the CMB structures.  The light is already on its way towards us but it will take a while to reach us since it will have to come from a sphere with a diameter of 93 billion light years.  This is the explanation for the difference; the “observable” universe is larger than we can see it today.

This is only, at best, a theoretical estimate of the diameter of the universe, though.  We now know that due to dark energy the expansion of the universe is accelerating.  This acceleration will not allow us to see those superclusters which are now 46.5 billion light years from us.  In fact, if we wait the requisite 32.7 billion years those superclusters will be receding from us at a rate that is greater than the speed of light.  We just will not be able to observe those galaxies and clusters of galaxies which have formed at the edge of the universe.

Jeff Mangum

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Academic Path to a Career in Astronomy

Question: Forgive me for asking a question that I’m sure you’ve seen many times over. I’m interested in radio telescopes and a career with them(particularly the prospect of working with the Square Kilometer Array). I’ve been contemplating an undergrad in physics with a possible double with chemistry (looking to go to a local university as I cannot afford going out of state for a university with an actual undergrad in astronomy or astrophysics) and then working towards an astrophysics graduate degree out of state. I’m thirty one and considered a sophomore. I’m finishing up my year in general chemistry and looking into calculus and physics starting the fall. Is shooting for an astrophysics degree a good way to go or should I go for an astronomy degree?

I’m trying to reach out into the field to get a better understanding now so I can make the right decisions in my academics. Thank you for taking the time to read this and if you do respond, I greatly appreciate that you took the time to do so. – Jonathan

Answer: At the graduate school level the astronomy and astrophysics degrees are effectively the same thing.  The difference is in name only.  You should choose a graduate school based on other factors like the research programs offered rather than whether the graduate degree is in astronomy or astrophysics.

Jeff Mangum

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Why Can’t Venus be Fully Lit by the Sun in the Ptolemaic System?

Question: The first known observations of the full planetary phases of Venus were by Galileo at the end of 1610 (though not published until 1613). Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system (which would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the sun, which is impossible if its orbit is, as the Ptolemaic system requires, between the Earth and the sun). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other geoheliocentric models. [From the Wikipedia page on the Phases of the Venus].

Can you please explain this?  In the Ptolemaic system if Venus orbits the earth why would it not be fully lit when it is on the far side of its orbit around the earth just like the moon when it is not between the earth and the Sun.  – Joe

Answer: In the Ptolemaic system the order of the planets, the Sun, and the stars from the Earth was as follows:

  1. Moon
  2. Mercury
  3. Venus
  4. Sun
  5. Mars
  6. Jupiter
  7. Saturn
  8. Fixed Stars

A key fact in this discussion is the fact that in the Ptolemaic system Venus is always between us and the Sun.  Therefore, we would always see some of the night-side of Venus (the side which faces away from the Sun).  This arrangement would never allow Venus to be fully-lit.  Therefore, the fat that Galileo observed the full-phase of Venus required that Venus orbited the Sun between the Earth and the Sun.

Jeff Mangum


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A Career as an Astronomer

Question: I am 15 and I plan to become and astronomer. I’m a freshman in high school. What classes do you suggest I take my sophomore year? Also, do astronomers travel a lot? What do you suggest I do to learn hands-on about astronomy.  – Anthony

Answer: You should check out my Careers in Astronomy page on this blog for information on various aspects of a career choice as an astronomer.   Regarding the classes that one should take in high school to prepare for a career as an astronomer, the best tip that I could give to an aspiring high school student who is interested in astronomy is to learn as much math and science as possible.  Astronomers are basically physicists, and math is the language of physics.  A good background in physics and math is essential for a productive career in astronomy.  Regarding travel, yes astronomers do travel a fair bit, but I would not say that it is excessive.  Mostly we travel to attend meetings or to conduct observations at observatories.  Finally, to learn hands-on astronomy your best bet is to try to find a summer internship working at a physics or astronomy research institute.  There are a growing number of these summer programs for high school students and they are a great way to “get your feet wet” working as an astronomer for a summer.

Jeff Mangum

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Dream Job Questionnaire

Question: Hello. For school, I have to interview a person with my dream job and I would be very grateful if you could answer some of my interview questions.  – Leo

Q1: What events in your life inspired you to become an astronomer?

I have always had an interest in science.  I am not sure what influenced my decision to pursue a career in astronomy, but I have also always been interested in astronomy.  I did also have very good science teachers in elementary and high school, so I believe that they were very influential in my career path.
Q2: How does the typical work day as an astronomer look like?

The great thing about astronomy is that it is rarely typical.  I have quite a wide range of tasks that I can work on during any given day.  Most of them involve solving astrophysical problems or working on keeping the National Radio Astronomy Observatory’s facilities running well.  All of them are interesting, though.
Q3: What college majors and degrees did you take to become an astronomer?

I have three degrees in astronomy.  Bachelors, Masters, and Doctorate.
Q4: What is the history of this occupation?

I think that astronomy must be one of the oldest of the sciences, dating from ancient times when people tried to figure out what stars where and why they moved the way they did.
Q5: How many years of college did you take to become an astronomer?

It took me four years to finish my bachelors degree, then two years to finish my Masters degree, followed by another three years to finish my Doctorate.
Q6: What is the main purpose of being an astronomer in your point of view and what accomplishments do they achieve?

The main purpose of astronomers is to figure out how the universe works.  Since many people find the study of planets, stars, and galaxies fascinating, astronomy also inspires people to better understand the physical world around them.  This results in an increased number of people who seek to understand science, and perhaps pursue careers in the sciences.
Q7: What is the best part or benefits for working as this occupation?

I think that the best part of my job is that I get to work on a variety of interesting problems.
Q8: Who in your life gave you the motivation and helped you to become an astronomer?

I think that my high school teachers were the main motivators for what became a career path in astronomy.
Q9: What is your specific role and what kind of astronomer are you?

I am a scientist at the National Radio Astronomy Observatory (NRAO) where I work to develop, update, and maintain radio telescope facilities.  NRAO’s telescopes are used by astronomers all over the world.
Q10: What is your name?

Jeff Mangum


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